It is well known that during the electrical charging of a zinc anode alkaline battery, zinc dendrites are formed, leading to the perforation of the separator that lays between the zinc anode and the charging electrode compartments, and therefore to a short-circuit, rendering the battery completely useless.
The typical solution is to use a rayon type or polymeric porous separator (e.g. anionic exchange membrane, polymeric film made by zirconia/polysulfone, polymeric films, cellulose films, etc.). Other typical solutions of separators are described in the paper “Battery Separators” from Pankaj Arora and Zhengming (John) Zhang published in Chemical Reviews, 2004, Vol. 104, No. 10, pages 4419-4462, herein incorporated by reference in its entirety.
However these separators are not totally satisfactory because they are too permeable to zincate ions and zinc dendrites are still formed and usually perforate the separator in less than 50 cycles.
One aim of the present patent application is to provide a solution to improve the characteristics of such separators and extend their service life.
Another aim of the invention is to provide a separator that can greatly enhance its ability to prevent dendrites perforation.
The invention relates to a separator which is permeable to hydroxide ion, for use in an electrically rechargeable electrochemical cell with alkaline electrolyte between a Zinc electrode and a counter electrode, wherein:                said separator containing at least one Dendrite Stopping Substance or DSS, said DSS being a metal-containing compound such as oxide, hydroxide, salt, etc . . . , said metal chosen from the metals of the group consisting of Pt, Pd, Ni, Fe and Mn, and        said DSS being in a form of particles.        
The inventor noticed that incorporating some “Dendrite Stopping Substance” (DSS) particles in the separator's material or pores reduces dendrite formation.
In the invention “particle” means an aggregate of material whose size is below 500 μm.
The separator of the invention is such that during charging, i.e. when hydroxide ions are flowing from the zinc electrode towards the counter electrode, the dendrites growing from the Zinc electrode towards the counter electrode are stopped in their progression through the separator when they encounter DSS particles of the said separator.
Separators according to the invention can be prepared by including DSS particles within the material of the separator when it is formed.
In other words, the DSS particles are added to the commonly used materials that are used to prepare separators.
Separators according to the invention can also be prepared by impregnating, with DSS particles, porous or fibrous typical material that are used to form separators. Classical alkaline-stable separators (known in the art) are for instance made of Polypropylene (PP)/Polyvinyl chloride (PVC), or Polytetrafluoroethylene (PTFE) or mixtures thereof. An alkaline-stable microporous separator according to the invention can be a microporous PVC separator for use in lead-acid batteries having a 45% to 70% volumetric porosity, a 0.05 μm to 10 μm pore size and a 0.1 to 1 mm thickness. The above-mentioned separator that contains the DSS particles can be obtained, for example, by soaking a classical separator with a DSS precursor, which results in DSS particles precipitation.
In an advantageous embodiment, the invention relates to the separator mentioned above, wherein said DSS is Ni(OH)2 or one of its precursor, such as NiCO3 or NiSO4.
In another embodiment, the invention relates to the separator mentioned above, wherein the size of the DSS particles is below 250 μm whereby minimizing the cost of the DSS material. It is to be noticed that larger particles would use more material and be more expensive for achieving the same effect.
In another advantageous embodiment, the invention relates to the separator mentioned above, wherein the thickness of the separator is ranging from 50 to 1000 μm whereby the dendrite stopping effect remains. A thicker separator would increase internal resistivity, would use more material and would be more expensive, while a thinner separator would have less effect to stop dendrites.
The invention also relates to an electrically rechargeable battery, with strong alkaline electrolyte, comprising at least one separator according to the invention, as mentioned above, said separator being placed between a Zinc electrode and a counter electrode.
In the invention a “strong alkaline electrolyte” means an electrolyte having a pH of at least 12.
In some embodiments the above mentioned battery comprises, as a cathode, an air electrode.
Moreover, in some embodiments the above mentioned battery comprises, as cathode, a nickel-oxide or a silver electrode.
The present invention provides separators that are comparatively more resistant to dendrites perforation and provides battery with longer service life than the one not comprising DSS particles.
This invention is advantageously suitable for zinc-nickel, zinc-silver, zinc-air, lithium-air electrically rechargeable batteries, especially for their use in stationery applications or electric vehicles such as e-cars, e-autobuses, e-trucks, e-bicycles, etc. Thus, the invention also relates to an electric or hybrid vehicle comprising at least one electrically rechargeable battery as mentioned above.
Another aspect of the invention relates to a method for preparing a separator as defined above, comprising:                a step of soaking a porous or fibrous material with a DSS precursor, such as NiCO3 or NiSO4 and        a step of having DSS particles precipitate within the said material.        
According to this process, separator commonly used for batteries is soaked into a DSS precursor. The DSS material then precipitates onto the cavities or the pores of the porous or fibrous material, conferring to the resulting material Dendrite Stopping properties.
Another aspect of the invention relates to a method for manufacturing a separator having Dendrite Stopping properties as defined above, comprising a step of adding DSS particles into the mixture of material used to prepare a separator
This process allows to manufacture a separator that directly contains DSS particles, and exhibits Dendrite Stopping properties.
When preparing such a separator, it is ready to use and does not need to be soaked with a DSS precursor.
The invention also relates to the use of a separator as mentioned above, for manufacturing an electric battery.
The invention will be further understood from the following detailed description of specific embodiments. The products and methods of the invention are illustrated but not limited to the following examples.